Lasers are used for a wide variety of applications including materials processing such as cutting, welding, drilling etc. More recently, optical fibre lasers have been developed where the active lasing element has been introduced into the core of the optical fibre itself and the pumping means necessary for establishing the laser action is introduced via the fibre cladding. Pumping energy (generally light) is applied via the waveguide formed by the inner and outer claddings and the core of the optical fibre is doped with a laser element such as a rare earth element. Pump light is absorbed by the active element in the core when rays of the cladding pump light intersect it. A laser oscillator is defined by two refractive index grating reflectors (Bragg grating) at either end of the fibre which is doped with the laser active iron (rare earth element).
The key elements of such a fibre are therefore the pump source, pump coupling and a cladding pumped fibre. Alternatively, a fibre laser may be formed by using as an input a seed source such as a semi-conductor laser diode or another fibre laser and using this to seed a series of optical fibre amplifiers.
High power fibre lasers of this type have applications in materials processing and also in telecommunications. To date, high power cladding pumped fibre lasers have concentrated on single-transverse-mode operation of the optical fibre gain medium. This has the advantage that the laser can have very good beam quality. However, there is a drawback with this scheme that, due to the small diameter of the core of the fibre that is required for single-transverse-mode operation, very high brightness pump sources (typically semi-conductor lasers) are required.
There is a need, therefore, for an improved cladding pumped optical fibre laser that overcomes the above limitations.